Composite indium oxide particle, method for producing same, conductive coating material, conductive coating film, and conductive sheet

ABSTRACT

An aqueous alkaline solution containing a tin salt dissolved therein is mixed with a zinc compound, and an aqueous solution of an indium salt is added to the mixture. The resultant hydroxide or hydrate containing tin, indium and zinc is treated by heating at a temperature of 110 to 300° C. in the present of water. Then, the resultant product is filtered, dried and treated by heating at a temperature of 300 to 1,000° C. in an air and further reduced at a temperature of 150 to 400° C. under a reducing atmosphere to obtain composite indium oxide particles of zinc oxide and tin-containing indium oxide, which have an average particle size of 5 to 100 nm. The resultant composite particles of zinc oxide and tin-containing indium oxide are suitably used to form a transparent conductive coating film having a UV-shielding effect.

FIELD OF THE INVENTION

The present invention relates to composite indium oxide particlescomprising zinc oxide and tin-containing indium oxide and a process formanufacturing the same, and a conductive coating composition, aconductive coating film and a conductive sheet, each comprising suchcomposite particles.

BACKGROUND OF THE INVENTION

As materials for transparent conductive coating compositions, there areknown tin oxide particles, antimony-containing tin oxide particles,tin-containing indium oxide particles, zinc oxide particles substitutedby aluminum, etc. Among these materials, tin-containing indium oxideparticles are used for coatings applied to the screens of cathode-raytubes (CRT) and liquid crystal displays (LCD) which are required to haveantistatic properties and electromagnetic wave-shielding properties,because the tin-containing indium oxide particles have high translucencyto visible light and high electric conductivity. Further, sheets havingthe tin-containing indium oxide particles dispersed and applied thereonare used for not only displays but also a wide variety of otherapplications such as touch panels, because of their translucency andconductivity.

However, the properties of coating films comprising the tin-containingindium oxide particles are inferior to tin-containing indium oxide filmsformed by a vapor deposition process or a sputtering process, and thus,the application of such coating films has been limited, because theadvantage that the coating films can be formed by a relatively simpleand inexpensive method, i.e., coating, has not been fully utilized. Thetin-containing indium oxide particles have another problem in the highercost of raw materials since indium as a main raw material is expensive.

On the other hand, zinc oxide particles, titanium oxide particles,cerium oxide particles, iron oxide particles, etc. are known asmaterials for UV-shielding or highly refractive coating compositions.Among these materials, zinc oxide particles show superior shieldingproperties to UV rays within the region of UV-A, and particularly showhigh transparency to visible light. Therefore, the zinc oxide particlesare used as UV-shielding cosmetic materials and are further used ashighly refractive materials because of their high refractive index(2.1).

When these transparent particles such as tin-containing indium oxideparticles and zinc oxide particles are dispersed in a binder forapplication, it is needed that the particle size thereof should begenerally at most a half of the wavelength of visible light in order toobtain high transparency to visible light. Accordingly, in order to betransparent to, for example, visible light, these particles should havea particle size of as small as 200 nm or less.

One of typical methods for manufacturing such fine particles isdisclosed in JP-A-62-7627. According to this method, an aqueous alkalinesolution such as an aqueous ammonia, an aqueous ammonium carbonatesolution or the like is added to an aqueous solution of a mixture ofindium chloride and tin chloride to form a co-precipitated hydroxide;the co-precipitated hydroxide is then treated by heating to formtin-containing indium oxide; the tin-containing indium oxide ismechanically ground to obtain fine particles thereof. In the method ofJP-A-62-7627, tin-containing indium oxide particles having an averageparticle size of 0.1 μm are obtained by the heat treatment and themechanical grinding.

According to JP-A-5-201731, the co-precipitated hydroxide of indium andtin is obtained in the same manner as in JP-A-62-7627, and then is bakedand ground to obtain tin-containing indium oxide particles, while it isimportant in this method that the contents of sodium and potassiumshould not be larger than a specified amount in order that the resultantparticles can have high conductivity. In the method of JP-A-5-201731,tin-containing indium oxide particles having a particle size of 0.01 to0.03 μm are obtained after the grinding.

On the other hand, it is known that zinc oxide fine particles themselveshave strong coagulating power and thus are hard to disperse. In order toimprove the dispersibility of zinc oxide fine particles, a very smallamount of an oxide or an hydroxide of silicon or aluminum is containedinto the respective zinc oxide particles (see JP-A-201382). Thereby,zinc oxide particles having a particle size of not larger than 0.03 μmand having sufficient dispersibility are obtained.

For example, these transparent conductive particles and UV-shielding andhighly refractive particles are used for anti-reflection films havingexcellent antistatic effect. While a conventional anti-reflection filmof this type is obtained by laminating a plurality of layers havingindividual functions, recently, an anti-reflection film having aplurality of functions in a single layer structure is desired inassociation with the development of a variety of thin-shaped appliances.For example, JP-A-2002-16757 discloses such a single-layer film. In thispublication, a highly refractive and conductive material is obtained bydispersing, in a binder, conductive fine particles which comprise indiumoxide and tin oxide as main components and highly refractive particleswhich comprise titanium oxide and zinc oxide. In this method, it isnecessary that the sizes of the particles should not be larger than 0.2μm, and the thickness of a coating film should not be larger than 20 μmin order to maintain the transparency of the film and the dispersibilityof the particles. In Example 1 of this publication, a coatingcomposition is prepared by mixing and dispersing tin-containing indiumoxide particles and cerium oxide particles, and a coating film having arefractive index of 1.68 and a surface resistance of 2.5×10⁹ Ω/□ isformed by applying this coating composition.

In the above mixture dispersion system of conductive particles andnon-conductive particles, the non-conductive particles are held betweenthe conductive particles by mixing and dispersing the non-conductiveparticles and the conductive particles, so that the contacts betweeneach of the conductive particles are decreased. As a result, theelectric conductivity of the resultant coating film tends to lower. Thisis one of the essential features of the mixture dispersion systems ofthis type. As for the antistatic effect alone, a coating film having asurface resistivity of about 10⁹ Ω/□ has sufficient conductivity, inother words, an antistatic function, and is expected to have furtherfunctions such as an electromagnetic wave-shielding property, orconductivity so high as to be applicable to a touch panel or the like.However, to obtain such excellent conductivity, it is needed to increasethe content of conductive particles of tin-containing indium oxide orthe like as much as possible. However, disadvantageously, an increase ofthe content of the conductive particles leads to a decrease of thecontent of non-conductive particles such as zinc oxide or the like. As aresult, the UV-shielding effect, i.e., one of the features of the zincoxide is hardly exhibited. In this way, there is a relationship oftrade-off between the conductivity and the UV-shielding function.

In these years, there is an increased demand for antistatic coatingcompositions comprising white conductive particles, for use in whitegarment and the interior decoration of clean rooms required to havecleanliness, or bright and vivid color coating compositions or plasticproducts. Examples of the white conductive particles for use in theseapplications are white inorganic pigment particles each of whichcomprises a core of potashmica coated with tin-containing indium oxide(JP-A-60-253112), white conductive particles each of which comprises awhite inorganic pigment particle of zinc oxide, titanium oxide or thelike, coated with tin dioxide and further coated with tin-containingindium oxide (JP-A-06-338213), etc. Some of these white conductiveparticles are manufactured by coating the outer surfaces of UV-shieldingwhite pigment particles of titanium oxide or zinc oxide as cores withconductive tin oxide, tin-containing indium oxide or the like. Thesesparticles are manufactured for the purpose of obtaining white colors butare not used as UV-shielding agents. Further, because of the influenceof the white inorganic pigment particles as base substances, it isdifficult to sufficiently lower the volume resistivity of the particlesto a level necessary for use in conductive coating composition or thelike.

DISCLOSURE OF THE INVENTION

To form a coating film containing the above-described metal oxideparticles, usually, the particles of this kind are dispersed in asolvent containing an inorganic or organic binder dissolved therein, andthe resultant coating composition is applied on a variety of substrates.In order for the resultant coating film to have transparency, theparticles to be used should be very fine, and simultaneously should beuniformly dispersed in the coating film. However, such fine particlestend to form secondary coagulations in the coating composition, whichmakes it difficult to obtain an uniform dispersion. Such coagulations inthe coating composition not only lower the conductivity of the resultantcoating film but also degrade the transparency thereof.

Under the above-discussed circumstances, an object of the presentinvention is to provide composite indium oxide particles havingexcellent dispersibility, as novel fine particles having transparency,conductivity and UV-shielding properties, for use in coating films, andfurther as fine particles for use in coating type transparent andconductive films which can be manufactured at lower cost by decreasingthe amount of indium, and another object of the invention is to providemethods for manufacturing the same. A further object of the presentinvention is to provide a conductive sheet or the like having excellenttransparency, conductivity and UV-shielding properties, by using thesame composite indium oxide particles.

As a result of the present inventors' extensive researches for achievingthe above objects, they have found the following: composite indium oxideparticles having an average particle size of 5 to 100 nm, whichsubstantially consist of zinc oxide and tin-containing indium oxide andwhich is manufactured by coating the zinc oxide as a core with thetin-containing indium oxide, have transparency and conductivity derivedfrom the tin-containing indium oxide particles, and UV-shieldingproperties derived from the zinc oxide, and further excellentdispersibility, in spite of the fact that the amount of indium used issmaller than that used in any of the conventional tin-containing indiumoxide particles; namely, fine particle materials suitable for use incoating films or coating type transparent conductive films can beobtained. The use of the particles having an average particle size ofless than 5 nm is undesirable, because such particles are hard todisperse in the preparation of a coating composition by using the same,and because the manufacturing of such particles is difficult. The use ofthe particles having an average particle size exceeding 100 nm is alsoundesirable, because the resultant coating film is hard to ensuretransparency to visible light.

Preferably, the composite indium oxide particles of the presentinvention are composite particles of zinc oxide and tin-containingindium oxide in which the zinc oxide and the tin-containing indium oxidekeep their own crystalline structures, respectively. Preferably,compressed powder of such composite particles shows a volume resistivityof 10⁻³ to 10 Ωcm under a pressure of 14.7 MPa (150 kgf/cm²).

The present inventors further have found that the conductivity of thecomposite indium oxide particles can be further improved by a unique andspecific method which the present inventors have developed bythemselves, wherein a part of at least one metal of the constituentmetals of the tin-containing indium oxide and the zinc oxide issubstituted by a trivalent element such as aluminum, gallium, boron,thallium or the like, preferably aluminum or gallium, and such atrivalent element is contained in the composite particle to obtain acomposite indium oxide particle of the chemical formula:[(Zn_(1-y)A_(y))O]_(1-x)[(ITO)_(1-z)A_(z)]_(x)wherein A represents a trivalent element such as Al, Ga or the like, andthat this composite indium oxide particle is further improved inconductivity.

Furthermore, the present inventors have succeeded in manufacturing ofcomposite indium oxide particles which have an average particle size of5 to 100 nm as mentioned above, and which comprise zinc oxide andtin-containing indium oxide, by a novel manufacturing process which isquite different from any of the conventional processes.

The process of the present invention comprises the steps of preparing anaqueous alkaline solution by adding a zinc compound (or a zinc compoundelementarily substituted by aluminum, gallium or the like) to an aqueousalkaline solution containing a tin salt dissolved therein, and, if thetin-containing indium oxide phase is elementarily substituted byaluminum, gallium or the like, adding an aluminum compound, a galliumcompound or the like thereto; adding an aqueous solution of an indiumsalt to the above aqueous alkaline solution; adjusting the pH of theresultant suspension containing precipitates of a zinc compound coatedwith a hydroxide or hydrate of tin and indium, to 4 to 12; treating thesuspension by heating at a temperature of 110 to 300° C. in the presenceof water; filtering and drying the resultant particles; treating theresultant particles by heating at a temperature of 300 to 1,000° C. inan air; and reducing the resultant particles at a temperature of 150 to400° C. under a reducing atmosphere. Thus, the above composite indiumoxide particles which are composite particles of zinc oxide andtin-containing indium oxide are manufactured.

The composite indium oxide particles obtained by the process of thepresent invention have a significant feature in that they haveUV-shielding properties in addition to transparency and conductivity.This is considered to be induced as follows: by coating zinc oxide withtin-containing indium oxide, electrons flow along the tin-containingindium oxide formed on the surfaces of the particles so that theexcellent conductivity of the tin-containing indium oxide is exhibited,and light passes through a whole of the particles, so that the excellentUV-shielding property of the zinc oxide is exhibited. As a result, itbecomes possible for one kind of the particle to concurrently exhibittransparency, conductivity and UV-shielding property.

Furthermore, the composite particles of the present invention can bemanufactured at lower cost, because indium is used in a smaller amount.The conventional tin-containing indium oxide particles as transparentconductive materials are expensive, and thus have not been widely used,whereas the composite particles of the present invention can bemanufactured at lower cost since the amount of indium can be decreasedby 20 to 80%, while the conductivity thereof is being kept equal to orhigher than that of the conventional tin-containing indium oxideparticles.

According to the present invention, by compounding zinc oxide andtin-containing indium oxide (more preferably by coating zinc oxide withtin-containing indium oxide), fine particles having transparency andconductivity derived from the tin-containing indium oxide and alsoUV-shielding properties and transparency derived from the zinc oxide canbe provided. In this case, by substituting any of the metal elements ofthe zinc oxide and the tin-containing indium oxide with aluminum,gallium or other element, the composite particles can have improvedconductivity while maintaining the transparency and the UV-shieldingproperties. This feature can not be obtained from the conventionalparticles as a mixture of transparent conductive particles andUV-shielding particles, and this feature can be firstly realized by thecomposite particles of the present invention. The composite indium oxideparticles obtained in the present invention are most suitable asconductive particles for use in transparent conductive coating filmscapable of exhibiting UV-shielding effect. In this sense, theindustrially applicable values of the present invention are verysignificant.

As described above, the composite indium oxide particles of the presentinvention can be obtained by compounding zinc oxide and tin-containingindium oxide (more preferably compounding by coating zinc oxide withtin-containing indium oxide). Thereby, the amount of indium used can beremarkably decreased, and the cost can be decreased by the decreasedamount of indium. In addition, the composite indium oxide particles ofthe present invention have excellent UV-shielding properties which arethe characteristics of zinc oxide, while maintaining excellentconductivity. Accordingly, coating compositions of the present inventioncomprising such composite particles, coating films of the presentinvention formed of the same coating compositions, and conductive sheetsof the present invention obtained by forming the same coating films onthe surfaces of transparent sheet-form substrates have excellenttransparency, conductivity and UV-shielding properties.

According to the manufacturing process of the present invention, theabove mentioned composite indium oxide particles having an averageparticle size of 5 to 100 nm can be obtained, wherein each compositeparticle comprises zinc oxide coated with tin-containing indium oxide.Such composite indium oxide particles are novel functional particleswhich have been firstly developed in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the X-ray diffraction spectrum of the composite indiumoxide particles of zinc oxide and tin-containing indium oxide, obtainedin Example 1, which contained zinc oxide at a concentration of 50 mol %.

FIG. 2 shows the transmission electron micro-photograph (magnification:300,000) of the composite particles of zinc oxide and tin-containingindium oxide, obtained in Example 1, which contained zinc oxide at aconcentration of 50 mol %.

FIG. 3 shows the transmission electron micro-photograph (magnification:300,000) of the composite particles of zinc oxide and tin-containingindium oxide, obtained in Example 6, which contained zinc oxide at aconcentration of 18 mol % and aluminum at a concentration of 5.2 mol %.

FIG. 4 shows the spectrum of light transmission (wavelength: 200 to2,500 nm) of the composite particles of zinc oxide and tin-containingindium oxide, obtained in Example 5, which contained zinc oxide at aconcentration of 45 mol % and aluminum at a concentration of 7 mol %.

MOST PREFERRED EMBODIMENT FOR WORKING THE INVENTION

The manufacturing process of the present invention is carried out asfollows: in the first step, a zinc compound, or a zinc compoundsubstituted by an element such as aluminum, gallium or the like is addedand mixed into an aqueous alkaline solution containing a tin saltpreviously dissolved therein; and an aqueous solution of an indium saltis added to the aqueous solution of the tin salt which contains the zinccompound. When the tin-containing indium oxide phase in a compositeindium oxide particle as a final product is substituted by an elementsuch as aluminum, gallium or the like, an aluminum compound, a galliumcompound or the like is further added and mixed to prepare an aqueousalkaline solution, to which an aqueous solution of an indium salt isadded. In this step, a zinc compound coated with a hydroxide or hydrateof tin and indium is obtained. Preferably, by adding an indium salt, thepH of a suspension containing precipitates of the zinc compound coatedwith the hydroxide or hydrate of tin and indium is adjusted to 4 to 12.

Next, this zinc compound coated with the hydroxide or hydrate of tin andindium is treated in an autoclave or the like by heating at atemperature of 110 to 300° C. in the presence of water, to regulate theparticles to a desired particle shape with a desired particle size.

In the second step, the zinc compound coated with the hydroxide orhydrate of tin and indium is treated by heating in an air so as to bereduced with hydrogen gas. Thus, composite indium oxide particles whichhave a uniform particle size distribution and which are hardly sinteredor coagulated are obtained.

The present inventors have firstly succeeded in the development of thecomposite indium oxide particles having an average particle size of 5 to100 nm, based on an inspired conception that the step for regulating theshapes and particle sizes of the particles is carried out separatelyfrom the step for extracting the intrinsic physical properties of thematerials as much as possible, in the course of manufacturing thecomposite indium oxide particles, i.e., the composite particles of zincoxide and tin-containing indium oxide.

A coating film comprising the composite indium oxide particles of thepresent invention manufactured by the above steps exhibits excellentperformance, i.e., concurrently have high transparency and conductivity,and UV-shielding properties, which have not been achieved by any of theconventional coating films.

Hereinafter, the process for manufacturing the composite indium oxideparticles of the present invention will be described in more detail.

(Formation of Precipitates)

An indium salt such as indium chloride, indium nitrate, indium sulfateor the like is dissolved in water to prepare an aqueous solutioncontaining indium ions. Among these indium salts, indium chloride ismost preferable to obtain fine composite indium oxide particles.

Separately, an alkaline solution containing tin ions is mixed with azinc compound or an elementarily substituted zinc compound to prepare asolution. In detail, an alkali such as sodium hydroxide, potassiumhydroxide, lithium hydroxide, an aqueous ammonia solution or the like isdissolved in water, and then, a tin salt such as tin chloride, tinnitrate, tin sulfate or the like is dissolved in the resultant aqueousalkaline solution. Further, a zinc compound such as zinc hydroxide, zincoxide or the like, or an elementarily substituted zinc compound such aszinc hydroxide, zinc oxide or the like substituted by a trivalentelement such as aluminum, gallium or the like is mixed with the abovesolution to prepare an alkaline solution. Among the above tin salts, tinchloride is most preferable to obtain fine composite indium oxideparticles.

In this step, this alkaline solution may be mixed with an aluminumcompound such as aluminum hydroxide, aluminum oxide or the like, or agallium compound such as gallium hydroxide, gallium oxide or the like.By this mixing, a part of the tin atoms or indium atoms in thetin-containing indium oxide phase in the composite indium oxideparticles as a final product are substituted by a trivalent element suchas aluminum, gallium or the like, to further improve the conductivity ofthe particles.

To improve the conductivity and the UV-shielding effect of the compositeindium oxide particles as the final product, the concentration of thezinc compound to be mixed is controlled so that the concentration of thezinc oxide in the composite indium oxide particles can be 5 to 70 mol %,preferably 10 to 50 mol %, and that the concentration of the tin in thetin-containing indium oxide phase can be preferably 3 to 30 mol %, morepreferably 5 to 15 mol %.

To further improve the conductivity of the composite particles, theconcentration of the trivalent element in the composite indium oxideparticles is preferably not larger than 30 mol %, when the trivalentelement such as aluminum, gallium or the like is contained in the zincoxide phase by way of substitution. Specifically, the concentration ofthe trivalent element in the zinc oxide phase is 0 to 30 mol %,preferably 2 to 15 mol % based on the mole of the zinc oxide, and theconcentration of the trivalent element in the tin-containing indiumoxide phase is 0 to 10 mol %, preferably 2 to 5 mol % based on the moleof the tin-containing indium oxide. In this regard, the wording “0 to Xmol %” seen in the description and the claims of the presentspecification means that the lower limit “0 mol %” is not included, andthat the upper limit “X mol %” is included, unless otherwise specified.

This is described below. In the composite indium oxide particlerepresented by the formula:[(Zn_(1-y)A_(y))O]_(1-x)[(ITO)_(1-z)A_(z)]_(x), the respectiveconcentrations are preferably controlled as follows: the concentrationof the zinc oxide or the elementarily substituted zinc oxide (theconcentration being represented by “1−x” in the above formula) is 5 to70 mol %; the total content of the element A which is a trivalentelement such as aluminum, gallium or the like, and which is contained inthe zinc oxide and the tin-containing indium oxide by way of thesubstitution is not larger than 30 mol %; the concentration of theelement A in the zinc oxide (represented by “y” in the above formula) is0 to 30 mol %; the concentration of the element A in the tin-containingindium oxide (represented by “z” in the above formula) is 0 to 10 mol %;and the concentration of tin in ITO is 3 to 30 mol %. In this case, theconcentration of the zinc oxide, (1-x), smaller than the lower limit ofthe above range does not influence the conductivity but degrades theUV-shielding effect. This concentration larger than the upper limit ofthe above range does not influence the UV-shielding effect but markedlydegrades the conductivity. When the total content of the trivalentelement(s) to be added to improve the conductivity is larger than theabove specified value, the conductivity of the composite particles, onthe contrary, tends to lower, or the trivalent elements separate sincethey are not completely used for the substitution. As a result, forexample, aluminum oxide is formed in the final product, so thatparticles of a mixture are provided. The concentration of tin in thetin-containing indium oxide smaller than the lower limit of the aboverange makes it impossible to obtain sufficient conductivity. On thecontrary, the concentration of tin in the tin-containing indium oxidelarger than the upper limit of the above range impairs the conductivity.

To contain the trivalent element in the composite particle, at least oneoxide of the internal zinc oxide and the tin-containing indium oxideformed on the surface of the zinc oxide is substituted by this trivalentelement to introduce the trivalent element into the above oxide. To moreeffectively improve the conductivity, the trivalent elements arecontained in both the oxides by way of substitution. Preferably,aluminum is contained in the composite particle as the trivalentelement.

Hydroxyalkylamine may be added to the above alkaline solution.Hydroxyalkylamine acts as a pH buffer within the alkaline region, andsimultaneously acts as a crystal growth-controlling agent. Examples ofhydroxyalkylamine for use in this case include monoethanolamine,triethanolamine, isobutanolamine, propanolamine and the like, amongwhich monoethanolamine acting also as a crystal growth-inhibiting agentis most preferable to obtain fine particles.

Next, the above aqueous solution of indium salt is added dropwise to theabove alkaline solution to form a zinc compound (precipitate) coatedwith a hydroxide or hydrate of tin and indium. The pH of the suspensioncontaining this precipitate is adjusted to 4 to 12, and the resultingsuspension is preferably aged at a temperature of 10 to 50° C. for 10 to100 hours. The pH adjustment and the aging are effective to obtaincomposite indium oxide particles at a relatively low treatingtemperature in the subsequent hydrothermal treatment. When the agingtime is shorter than 10 hours, the effect of aging is poor. On the otherhand, when the aging time is longer than 100 hours, so serious influenceis not given the suspension, but such long aging is meaningless sincethe effect of aging is saturated.

(Hydrothermal Treatment)

The suspension containing the zinc compound coated with the hydroxide orhydrate of tin and indium is subjected to a hydrothermal treatment in anautoclave or the like. By washing the suspension containing the aboveprecipitate with water in this hydrothermal treatment, products andresidues other than the precipitate are removed. After that, the pH ofthe suspension may be again adjusted with NaOH or the like. The pH ofthe suspension in this adjustment is preferably within a range of 4 to12. When the pH is lower than the lower limit, tin hydroxide is againdissolved in the hydrothermal treatment. When the pH is too high, theparticle size distribution of the resultant particles becomes wider, ortin, zinc and indium and further aluminum are not compounded by there-dissolution thereof, so that a compound may be formed.

The temperature for the hydrothermal treatment is preferably in therange of 110 to 300° C. When this temperature is lower than the lowerlimit, sufficient crystal growth of the particles can not be expected.When the temperature is higher than the upper limit, a generatedpressure becomes higher for which an expensive apparatus is needed towithstand such a high pressure, resulting in no advantage.

The time required for the hydrothermal treatment is preferably in therange of 1 to 4 hours. When this time is too short, the crystal growthof the particles becomes insufficient. The too long hydrothermaltreatment is no use, resulting in higher manufacturing cost, although noparticular problem arises therefor.

(Heat Treatment)

The zinc compound coated with the hydroxide or hydrate of tin and indiumobtained after the hydrothermal treatment is filtered and dried and thenis subjected to a heat treatment. In this step, prior to the filtration,the zinc compound is preferably washed with water so as to adjust the pHof the compound to about 6 to about 9 within the neutral region. This isbecause the water-soluble Na ions, or the amine-containing substance, ifhydroxyalkyl-amine is added, are removed by the washing with water. Ifthe zinc compound retaining such Na ions or such an amine-containingsubstance is filtered and dried and then is subjected to a heattreatment, the conductivity of the resultant particles tend to lower,and therefore, it is desirable to remove the above Na ions, etc. as muchas possible.

Alternatively, the zinc compound coated with the hydroxide or hydrate oftin and indium may be further admixed with a silicon compound such assodium silicate or the like to carry out a silica treatment. This silicatreatment is effective for the composite indium oxide particles as thefinal product to be kept as fine particles. The composite indium oxideparticles are finally coated at their surfaces with silica films by thistreatment, and therefore, silica is effective to maintain the fineparticles. However, the silica films tend to lower the conductivity ofthe composite indium oxide particles. Therefore, the amount of thesilicon compound to be added is preferably 0.1 to 5 wt. % based on theweight of the composite indium oxide particles.

Next, the zinc compound coated with the hydroxide or hydrate of tin andindium, which has been filtered and dried, is subjected to a heattreatment to obtain composite indium oxide particles. This heattreatment is carried out under an atmosphere of an oxygen concentrationof at least 10%. The heating in an air is preferable since it costslowest. The temperature for the heat treatment is preferably in therange of 300 to 1,000° C. When this temperature is lower than 300° C.,the tin of the tin-containing indium oxide for coating is hardlysubstituted, so that the resultant particle has a mixture-like structureof tin oxide, indium oxide and zinc oxide, or that the hydroxide is leftas it is and is not transformed into an oxide. Under such a situation,the resultant particles are hard to obtain sufficient conductivity. Whenthis temperature is too high, the particles are easily sintered, andsuch particles are not sufficiently dispersed in the preparation of acoating composition. Further, when such a coating composition is used toform a coating film, the transparency of the resultant coating filmtends to degrade.

The composite indium oxide particles obtained after the heat treatmentare reduced in an atmosphere of 150 to 400° C. to obtain conductivecomposite indium oxide particles colored yellowish gray or blue-greenishwhite.

The composite indium oxide particles thus obtained have a particle sizeof 5 to 100 nm which is within a range particularly preferable for usein transparent conductive films. When the particle size is smaller than5 nm, the coagulating force among the particles becomes extremely largeso that the dispersion of such particles becomes hard. When the particlesize is larger than 100 nm, the transparency of the resultant coatingfilm obtained by applying a coating composition having such particlesdispersed therein tends to degrade. For these reasons, the averageparticle size of the composite indium oxide particles is preferably 5 to100 nm, more preferably 10 to 60 nm.

(Preparation of Coating Composition and Manufacturing of Sheet)

A conductive sheet according to the present invention is manufactured byapplying, to a sheet-shaped substrate, a conductive coating compositionwhich is prepared by adding the composite indium oxide particlesobtained as above to a solution containing a binder and a solvent, andmixing and dispersing them in each other. Hereinafter, the manufactureof this conductive sheet is described in detail.

For example, a resin film or sheet is used as the substrate. The resinfilm or sheet may be manufactured by any of known methods. For example,a resin such as a saturated polyester resin, polycarbonate resin,polyacrylate resin, alicyclic polyolefin resin, polystyrene resin,polyvinyl chloride resin, polyvinyl acetate resin or the like is formedinto a film or sheet by melt-molding such as extrusion molding, calendermolding, compression molding, injection molding or the like; orotherwise, the resin is dissolved in a solvent, and the solution of theresin is cast to form a film or sheet. The thickness of the substrate ispreferably about 10 μm to about 5 mm. In this regard, the substrate maycontain additives such as an antioxidant, a flame retardant, a heatresisting agent, a UV absorbent, a lubricant, an antistatic agent, etc.To improve the adhesion of the film, the substrate may be provided atits surface with an adhesive layer (or a primer) or may be subjected toany of known surface treatments such as corona treatment, plasmatreatment or the like.

A dispersing machine for dispersing the composite indium oxide particlesand a binder to form a dispersion is not particularly limited, and aknown dispersing machine such as a three-roll mill, a ball mill, a paintshaker, an ultrasonic dispersing machine or the like can be used.

The amount of the composite indium oxide particles in relation to thebinder is preferably 50 to 99%, more preferably 70 to 92%, in terms ofthe weight content of the solids in the coating composition (that is,preferably 100 to 9,900 wt. parts, more preferably 230 to 1,150 wt.parts of the composite indium oxide particles per 100 wt. parts of thebinder). When this weight content is lower than 50%, the probability ofthe mutual contact of the particles deteriorates, which makes it hard toform electrically conductive paths, so that the conductivity of theresultant film becomes lower, and the UV-shielding effect of the filmtends lower, since the particles can not have a sufficient physicalvolume in the film. On the other hand, when the weight content exceeds99%, the binding power of the binder to the particles becomes lower, sothat some of the particles drop or that the resultant film cracks, whichleads to a decrease in the adhesion between the coating film and thesubstrate. As a result, the conductivity of the coating film degrades.In the meantime, when other conductive substance (e.g., a metal such asAu or Ag, metal colloid, carbon nanotubes or the like) is added togetherwith the composite indium oxide particles to the binder, to an extentthat the optical properties of the film are not impaired, the weightcontent of all the conductive substances is also preferably within theabove specified range.

As the binder (a polymer binder), all kinds of binders, includingaqueous types and non-aqueous types, can be used, so long as thedispersion is not inhibited by any of such binders. Examples of thebinder include acrylic resins, polyurethanes, polyvinyl chloride, epoxyresins, polyesters, polyvinyl acetates, polystyrene, cellulose,polyvinyl alcohol and polybutyral, and resins produced by copolymerizingthese resins. If needed, there can be used, as the binder, a binderhaving functional groups, and conductive polymer materials such aspolythiophene derivatives (PEDOT),N,N-di(naphthalene-1-nyl)-N,N-diphenyl-benzidene derivatives (NPB),poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) andthe like. Further, a known dispersant, surfactant, leveling agent, etc.may be added to the binder, so as to improve the stability of theresultant coating composition, so long as the optical properties andconductivity of the film are not impaired by these additives.

Examples of the solvent for use in the conductive coating compositioninclude ketones such as acetone, methyl ethyl ketone, methyl isobutylketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuranand the like; alcohols such as methanol, ethanol, propanol, butanol,isobutanol, isopropanol, methylcyclohexanol and the like; esters such asmethyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate,ethyl lactate, glycol acetate and the like; glycol esters such asglycoldimethylether, glycolmonoethylether, dioxane and the like;aromatic hydrocarbons such as benzene, toluene, xylene, cresol,chlorobenzene and the like; chlorinated hydrocarbons such as methylenechloride, ethylene chloride, carbon tetrachloride, chloroform, ethylenechlorohydrine, dichlorobenzene and the like; N,N-dimethylformamide;hexane; and the like. Each of these solvents may be used alone or as amixture with others selected therefrom in an optional ratio.

A conductive sheet having an UV-shielding, transparent and conductivecoating film on its surface (a transparent conductive sheet in casewhere a transparent substrate is used) can be obtained by applying theabove coating composition comprising the composite indium oxideparticles, the binder and the solvent, on a sheet-form substrate. Thecoating composition may be applied by any of the known coating methods(roll coating, die coating, air knife coating, blade coating, spincoating, reverse coating, gravure coating, micro-gravure coating or thelike) or by any of known printing methods (gravure printing, screenprinting, off-set printing, ink jet printing or the like).

The dry thickness of the coating film is preferably 50 nm to 15 μm, morepreferably 500 nm to 5 μm. When this thickness is less than 50 nm, thecoating film can not obtain sufficient conductivity and UV-shieldingeffect. When it exceeds 15 μm, the transmittance of the coating film tovisible light becomes poor, so that, undesirably, the coating film tendsto have lower transparency or tends to crack.

The conductivity and visible light transmittance of the coating film canbe improved by calendering the coating film. In this case, thecalendering is carried out at a rate of 1 to 30 m/min. and at aheat-treating temperature within a range of Tg±50° C. (Tg: the glasstransition temperature of the resin used as the binder) under a pressureof 9.8×10⁴ to 9.8×10⁶ Pa (1 to 100 kg/cm²).

Hereinafter, the Examples of the present invention and ComparativeExamples will be illustrated. In the following Examples and ComparativeExamples, the unit “part(s)” indicates “wt. part(s)”, unless otherwisespecified.

EXAMPLE 1

(Preparation of Composite Indium Oxide Particles)

Sodium hydroxide (15.4 g) was dissolved in water (800 ml), and tin (IV)chloride pentahydrate (3 g) was dissolved in this alkaline solution toform an aqueous alkaline solution. This aqueous alkaline solution wasmixed with zinc oxide powder (13 g). Separately, indium (III) chloridetetrahydrate. (35 g) was dissolved in water (400 ml) to form an aqueoussolution of indium chloride. This aqueous solution of indium chloridewas added dropwise to the former aqueous alkaline solution containingtin ions and zinc oxide to form precipitates of zinc oxide coated withthe hydroxide or hydrate of tin and indium. The pH of the precipitateswas 8.8. The precipitates in the form of a suspension were aged at aroom temperature for about 20 hours.

Next, an aqueous solution of sodium hydroxide was added to thesuspension containing the precipitates to readjust the pH of thesuspension to 10.0, and this suspension was charged in an autoclave andwas subjected to a hydrothermal treatment at 180° C. for 4 hours.

The resulting product of the hydrothermal treatment was filtered andwashed until the pH thereof reached 7.8, and then was dried at 90° C. inan air. The dried product was then slightly crushed in a mortar andtreated by heating at 600° C. in an air for 2 hours, followed byreduction at 250° C. in a hydrogen atmosphere to obtain composite indiumoxide particles which were composite particles of zinc oxide andtin-containing indium oxide.

The shapes of the composite indium oxide particles thus obtained wereobserved with a transmission electron microscope, and the compositeparticles were found to have particle sizes of 20 to 30 nm. Thecomposition of the particles was represented by the formula:(ZnO)_(0.5)(ITO)_(0.5) deduced from the charged amounts. The contents ofzinc and tin in the composite particle found by fluorescent X-rayanalysis were 50 mol % and 5 mol %, respectively, and the concentrationof tin in ITO was 10 mol %.

FIG. 1 shows the X-ray diffraction spectrum of the composite indiumoxide particles, and FIG. 2 shows the transmission electronmicrophotograph of the same particles at a magnification of 300,000times. The particles shown in the transmission electron microphotographhave uniform shapes, while the X-ray diffraction spectrum indicates thatthe particle comprises two phases of indium oxide and zinc oxide, and itis understood that the tin-containing indium oxide coats the zinc oxide.

(Formation of Coating Film)

Next, a coating composition was prepared using the above compositeindium oxide particles. The following components were stirred and mixed,and the mixture was dispersed with a paint shaker for 25 minutes toprepare the coating composition. Composite indium oxide particles 85parts Polymethyl methacylate 15 parts (BR113 manufactured by MitsubishiRayon Co., Ltd.) Methyl ethyl ketone 50 parts Toluene 50 parts

The resultant coating composition was applied to a transparentsheet-form substrate made of polyethylene terephthalate (PET) with a barcoater so that the thickness of the resulting coating layer could have athickness of 3 μm after being dried. The coating layer was dried toobtain a conductive sheet having an UV-shielding conductive coating filmformed on its surface.

EXAMPLE 2

An aqueous alkaline solution was prepared by mixing a zinc oxideelementarily substituted by aluminum (10 mol % based on the mole ofzinc) instead of the zinc oxide, in the course of the synthesis of thecomposite indium oxide particles of Example 1. Except for this step,composite indium oxide particles were prepared in the same manners as inExample 1, that is, precipitates containing the zinc oxide elementarilysubstituted by aluminum, coated with the hydroxide or hydrate of tin andindium, were formed, washed with water, filtered and dried, followed bya heat treatment and reduction, to obtain the composite indium oxideparticles. The composition of this particle is represented by theformula: [(Zn_(0.9)Al_(0.1))O]_(0.5)(ITO)_(0.5) deduced from the chargedamounts. The contents of zinc, aluminum and tin in the particle, foundby fluorescent X-ray analysis, were 45 mol %, 5 mol % and 5 mol %,respectively, and the concentration of tin in ITO was 10 mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that the particle comprised two phases oftin-containing indium oxide and zinc oxide elementarily substituted byaluminum. The particles were observed with a transmission electronmicroscope, and it was found that the particles had uniform shapeshaving particle sizes of 20 to 30 nm, like those of Example 1. Fromthese facts, it was understood that the tin-containing indium oxidecoated the zinc oxide elementarily substituted by aluminum, like Example1.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 3

An aqueous alkaline solution was prepared by adding sodium hydroxide(24.7 g) to water (800 ml), and dissolving tin (IV) chloridepentahydrate (4.8 g) in this alkaline solution, in the course of thesynthesis of the composite indium oxide particles of Example 1. Thisaqueous alkaline solution was mixed with zinc oxide (5 g) elementarilysubstituted by aluminum (10 mol % based on the mole of zinc) instead ofthe zinc oxide powder (13 g). Separately, indium (III) chloridetetrahydrate (56.1 g) was dissolved in water (400 ml) to form an aqueoussolution of indium chloride. After that, composite indium oxideparticles were prepared in the same manners as in Example 1, that is,precipitates containing the zinc oxide elementarily substituted byaluminum, coated with the hydroxide or hydrate of tin and indium, wereformed, washed with water, filtered and dried, followed by a heattreatment and reduction, to obtain the composite indium oxide particles.The composition of this particle is represented by the formula:[(Zn_(0.9)Al_(0.1))O]_(0.2)(ITO)_(0.8) deduced from the charged amounts.The contents of zinc, aluminum and tin in the particle, found byfluorescent X-ray analysis, were 18 mol %, 2 mol % and 8 mol %,respectively, and the concentration of tin in ITO was 10 mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that the particle comprised two phases ofthe tin-containing indium oxide and the zinc oxide elementarilysubstituted by aluminum. The particles were observed with a transmissionelectron microscope, and it was found that the particles had uniformshapes having particle sizes of 20 to 30 nm, like those of Example 1.From these facts, it was understood that the tin-containing indium oxidecoated the zinc oxide elementarily substituted by aluminum, like Example1.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 4

An aqueous alkaline solution was prepared by mixing the alkalinesolution with aluminum hydroxide (0.32 g), in the course of thesynthesis of the composite indium oxide particles of Example 1. Exceptfor this step, composite indium oxide particles were prepared in thesame manners as in Example 1, that is, precipitates containing zincoxide coated with the hydroxide or hydrate of tin, indium and aluminumwere formed, aged, subjected to a hydrothermal treatment, filtered,washed and dried, followed by a heat treatment and reduction to obtainthe composite indium oxide particles. The composition of this particleis represented by the formula: (ZnO)_(0.5)[(ITO)_(0.96)Al_(0.04)]_(0.5)deduced from the charged amounts. The contents of zinc, aluminum and tinin the particle, found by fluorescent X-ray analysis, were 50 mol %, 2mol % and 4.8 mol %, respectively, and the concentration of tin in ITOwas 10 mol %.

The composite indium oxide particles were observed with a transmissionelectron microscope, and it was found that the particles had uniformshapes having particle sizes of 20 to 30 nm. The X-ray diffractionspectrum of the particles was obtained, and it was similar to that ofExample 1, and it was found that no independent peak assigned toaluminum was observed. From these facts, it was understood that thetin-containing indium oxide was elementarily substituted by aluminum,and that the zinc oxide was coated with the tin-containing indium oxidesubstituted by aluminum.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 5

An aqueous alkaline solution was prepared by mixing the alkalinesolution with zinc oxide elementarily substituted by 10 mol % ofaluminum instead of the zinc oxide, and further mixing with aluminumhydroxide (0.32 g), in the course of the synthesis of the compositeindium oxide particles of Example 1. Except for this step, compositeindium oxide particles were prepared in the same manners as in Example1, that is, precipitates containing the zinc oxide substituted byaluminum, coated with the hydroxide or hydrate of tin, indium andaluminum, were formed, aged, subjected to a hydrothermal treatment,filtered, washed and dried, followed by a heat treatment and reduction,to obtain the composite indium oxide particles. The composition of thisparticle is represented by the formula:[(Zn_(0.9)Al_(0.1))O]_(0.5)[(ITO)_(0.96)Al_(0.04)]_(0.5) deduced fromthe charged amounts. The contents of zinc, aluminum and tin in theparticle, found by fluorescent X-ray analysis, were 45 mol %, 7 mol %and 4.8 mol %, respectively, and the concentration of tin in ITO was 10mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was similar to that of Example 2 were obtained. Itwas found that no independent peak assigned to aluminum was observed.From these facts, it was understood that the particle comprised twophases of tin-containing indium oxide substituted by aluminum and zincoxide substituted by aluminum. The particles were further observed witha transmission electron microscope, and it was found that the particleshad uniform shapes having particle sizes of 20 to 30 nm. From thesefacts, it was understood that particles of zinc oxide substituted byaluminum, which were coated with indium oxide substituted by aluminum,were obtained.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 6

An aqueous alkaline solution was prepared by dissolving sodium hydroxide(24.7 g) in water (800 ml), and then dissolving tin (IV) chloridepentahydrate (4.8 g) in this alkaline solution, in the course of thesynthesis of the composite indium oxide particles of Example 1. Thisaqueous alkaline solution was mixed with zinc oxide (5 g) substituted byaluminum (10 mol % based on the mole of the zinc) instead of the zincoxide powder (13 g), and further mixed with aluminum hydroxide powder(0.51 g). Separately, an aqueous solution of indium chloride wasprepared by dissolving indium (III) chloride tetrahydrate (56.1 g) inwater (400 ml). After that, composite indium oxide particles wereprepared in the same manners as in Example 1, that is, precipitatescontaining the zinc oxide substituted by aluminum, coated with thehydroxide or hydrate of tin and indium, were formed, washed with water,filtered and dried, followed by a heat treatment and reduction, toobtain the composite indium oxide particles. The composition of thisparticle is represented by the formula:[(Zn_(0.9)Al_(0.1))O]_(0.2)[(ITO)_(0.96)Al_(0.04)]_(0.8) deduced fromthe charged amounts. The contents of zinc, aluminum and tin in theparticle, found by fluorescent X-ray analysis, were 18 mol %, 5.2 mol %and 7.7 mol %, respectively, and the concentration of tin in ITO was 10mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that the particle comprised two phases ofthe tin-containing indium oxide substituted by aluminum and the zincoxide substituted by aluminum. The particles were further observed witha transmission electron microscope, and it was found that the particleshad uniform shapes having particle sizes of 20 to 30 nm, like Example 1.From these facts, it was understood that the particles of the zinc oxidesubstituted by aluminum were coated with the tin-containing indium oxidesubstituted by aluminum. The transmission electron microphotograph ofthe composite indium oxide particles is shown in FIG. 3.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 7

In the course of the synthesis of the composite indium oxide particlesof Example 1, a solution of indium chloride was added dropwise to theaqueous alkaline solution containing tin ions and zinc oxide to formprecipitates containing the zinc oxide coated with the hydroxide orhydrate of tin and indium, and the precipitates in the form of asuspension were aged at 90° C., but not a room temperature, for about 20hours.

Next, an aqueous solution of sodium hydroxide was added to thesuspension of the precipitates to adjust the pH of the suspension to13.0 but not 10.0. This suspension was charged in an autoclave and wassubjected to a hydrothermal treatment at 180° C. for 4 hours. Except forthese steps, composite indium oxide particles were prepared in the samemanners as in Example 1, that is, precipitates containing the zinc oxidesubstituted by aluminum, coated with the hydroxide or hydrate of tin andindium, were formed, washed with water, filtered and dried, followed bya heat treatment and reduction, to obtain the composite indium oxideparticles. The composition of this particle, and the contents of zinc,aluminum and tin in the particle, found by the fluorescent X-rayanalysis, were the same as those found in Example 1.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that the particle comprised two phases ofthe tin-containing indium oxide and the zinc oxide substituted byaluminum, like Example 1. The particles were further observed with atransmission electron microscope, and it was found that the particleshad uniform shapes having particle sizes of 50 to 60 nm. From thesefacts, it was understood that the particles of the zinc oxidesubstituted by aluminum were coated with the tin-containing indiumoxide.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 8

In the course of the synthesis of the composite indium oxide particlesof Example 1, an aqueous alkaline solution was prepared by mixing thealkaline solution with zinc oxide substituted by 10 mol % of aluminumbut not with zinc oxide, and further mixed with aluminum hydroxide (0.32g). Precipitates containing zinc oxide substituted by aluminum, coatedwith the hydroxide or hydrate of tin, indium and aluminum, were formedin the same manner as in Example 1, and the pH of the suspension wasadjusted to 5.0. The suspension was aged at a room temperature for 20hours, and was subjected to a hydrothermal treatment under a conditionof pH 5.0, filtered, washed, dried, treated by heating and reduced toobtain composite indium oxide particles. The composition of thisparticle is represented by the formula:[(Zn_(0.9)Al_(0.1))O]_(0.5)[(ITO)_(0.96)Al_(0.04)]_(0.5) deduced fromthe charged amounts. The contents of zinc, aluminum and tin in theparticle, found by fluorescent X-ray analysis, were 45 mol %, 7 mol %and 4.8 mol %, respectively, and the concentration of tin in ITO was 10mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that a spectrum similar to that ofExample 2 was obtained. From the fact that no independent peak assignedto aluminum was observed, it was found that the particle comprised twophases of the tin-containing indium oxide substituted by aluminum andthe zinc oxide substituted by aluminum. The particles were furtherobserved with a transmission electron microscope, and it was found thatthe particles had uniform shapes having particle sizes of 40 to 50 nm.From these facts, it was understood that the particles of zinc oxidesubstituted by aluminum were coated with the tin-containing indium oxidesubstituted by aluminum.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

EXAMPLE 9

In the course of the synthesis of the composite indium oxide particlesof Example 6, precipitates containing zinc oxide substituted byaluminum, coated with the hydroxide or hydrate of tin and indium, wereformed, while maintaining the alkaline solution at 60° C., and the pH ofthe suspension was adjusted to 5.0. The suspension was aged at a roomtemperature, and was subjected to a hydrothermal treatment. Except forthese steps, the product obtained by the hydrothermal treatment waswashed with water, filtered, dried, treated by heating and reduced inthe same manners as in Example 6, to obtain composite indium oxideparticles. The composition of this particle is represented by theformula: [(Zn_(0.9)Al_(0.1))O]_(0.2)[(ITO)_(0.96)Al_(0.04)]_(0.8)deduced from the charged amounts. The contents of zinc, aluminum and tinin the particle, found by fluorescent X-ray analysis, were 18 mol %, 5.2mol % and 7.7 mol %, respectively, and the concentration of tin in ITOwas 10 mol %.

The X-ray diffraction spectrum of the composite indium oxide particleswas obtained, and it was found that the particle comprised two phases ofthe tin-containing indium oxide substituted by aluminum and zinc oxidesubstituted by aluminum. The particles were further observed with atransmission electron microscope, and it was found that the particleshad uniform shapes having particle sizes of 40 to 60 nm. From thesefacts, it was understood that the particles of the zinc oxidesubstituted by aluminum were coated with the tin-containing indium oxidesubstituted by aluminum.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

COMPARATIVE EXAMPLE 1

In the course of the synthesis of the composite indium oxide particlesof Example 1, a solution of indium chloride was added dropwise to analkaline solution containing tin chloride previously dissolved therein,and the resultant hydrate or hydroxide containing tin and indium wasaged and subjected to a hydrothermal treatment, and then was filtered,washed, dried, treated by heating and reduced in the same manners as inExample 1, to obtain tin-containing indium oxide particles.

The composition of the tin-containing indium oxide particles wasconfirmed from the result of the X-ray diffraction. The particles wereobserved with a transmission electron microscope, and were found to berectangular plate-shaped particles having particle sizes of 40 to 70 nm.The content of tin found by the fluorescent X-ray analysis was 10 mol %.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using these particles, in the same manner as in Example 1.

COMPARATIVE EXAMPLE 2

An aqueous alkaline solution was prepared by dissolving sodium hydroxide(53 g) in water (640 ml). Separately, an aqueous solution of zincchloride was prepared by dissolving zinc chloride (90 g) in water (320ml). The solution of zinc chloride was added dropwise to the formeraqueous alkaline solution to form precipitates containing a hydroxide orhydrate of zinc. The pH of the precipitates was 12.3. The precipitatesin the form of a suspension were aged at a room temperature for about 20hours, and then, an aqueous solution of sodium hydroxide was added toreadjust the pH of the suspension to 12.5. This suspension was chargedin an autoclave and subjected to a hydrothermal treatment at 180° C. for4 hours. The product obtained by the hydrothermal treatment was filteredand washed until the pH of the product reached 7.8. This product wasdried at 90° C. in an air, and then was slightly crushed in a mortar andtreated by heating at 600° C. in an air for 2 hours. Thus, zinc oxideparticles were obtained.

Next, the resultant zinc oxide particles were mixed with thetin-containing indium oxide particles obtained in Comparative Example 1,in a molar ratio of 1:1, to obtain mixed powder of the zinc oxideparticles and the tin-containing indium oxide particles. The contents ofthe constitutive elements found by the fluorescent X-ray analysis weresimilar to the results of Example 1.

Then, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the above mixed powder, in the same manner as inExample 1.

COMPARATIVE EXAMPLE 3

In the course of the synthesis of the composite indium oxide particlesof Example 1, an aqueous alkaline solution was prepared withoutdissolving tin chloride in the alkaline solution. Except for this step,composite indium oxide particles were manufactured in the same mannersas in Example 1, that is, precipitates containing zinc oxide coated witha hydroxide or hydrate of indium were formed, aged and subjected to ahydrothermal treatment, and the resultant product was filtered, washed,dried, treated by heating and reduced to obtain the composite indiumoxide particles. It is considered that the composition of this particleis represented by the formula: (ZnO)_(0.5)(InO_(3/2))_(0.5) deduced fromthe charged amounts. The content of zinc in the particle, found by thefluorescent X-ray analysis, was 50 mol %.

The composite indium oxide particles were observed with a transmissionelectron microscope, and it was found that the particles had uniformshapes having particle sizes of 20 to 30 nm. The X-ray diffractionspectrum of the composite indium oxide particles was obtained, and itwas found that the particle comprised two phases of zinc oxide andindium oxide.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using the composite indium oxide particles, in the samemanner as in Example 1.

COMPARATIVE EXAMPLE 4

In the course of the synthesis of the composite indium oxide particlesof Example 1, an aqueous alkaline solution was prepared by mixing withzinc oxide substituted by aluminum (40 mol %) instead of the zinc oxide,and further mixing with aluminum hydroxide (3.2 g). Except for thisstep, particles were manufactured in the same manners as in Example 1,that is, precipitates containing the zinc oxide substituted by aluminum,coated with a hydroxide or hydrate of tin and indium, were formed,washed with water, filtered, dried, treated by heating, and reduced toobtain the particles. The composition of this particle is represented bythe formula: [(Zn_(0.6)Al_(0.4))O]_(0.5)[(ITO)_(0.6)Al_(0.4)]_(0.5)deduced from the charged amounts. The contents of zinc, aluminum andtin, found by the fluorescent X-ray analysis, were 30 mol %, 40 mol %and 3 mol %, respectively, and the content of tin in ITO was 10 mol %.

The X-ray diffraction spectrum of the particles was obtained, and it wasfound that the particle comprised three separated phases of thetin-containing indium oxide, the zinc oxide substituted by aluminum, andadditionally, γ-aluminum oxide. The particles were observed with atransmission electron microscope, and it was found that the particlescomprised the particles having particle sizes of 20 to 30 nm as inExample 1, mixed with rectangular plate-shaped particles of, seemingly,aluminum oxide having particle sizes of 40 to 50 nm.

Next, a conductive sheet having a conductive coating film with athickness of 3 μm (in the dried state) formed on its surface wasmanufactured using these particles, in the same manner as in Example 1.

The structures of the particles obtained in Examples and ComparativeExamples are shown in Table 1. TABLE 1 Inner Composition Compositionsubstance/ of inner of coating Coating Inner substance: Coatingsubstance substance substance Al/Zn substance Al/Sn/In Ex. 1 50/50 ZnO0/100 ITO*¹⁾ 0/10/90 Ex. 2 50/50 AZO*²⁾ 10/90 ITO 0/10/90 Ex. 3 20/80AZO 10/90 ITO 0/10/90 Ex. 4 50/50 ZnO 0/100 Al-ITO*³⁾ 4/9.6/86.4 Ex. 550/50 AZO 10/90 Al-ITO 4/9.6/86.4 Ex. 6 20/80 AZO 10/90 Al-ITO4/9.6/86.4 Ex. 7 50/50 ZnO 0/100 ITO 0/10/90 Ex. 8 50/50 AZO 10/90Al-ITO 4/9.6/86.4 Ex. 9 20/80 AZO 10/90 Al-ITO 4/9.6/86.4 C. Ex. 1 ITO(Al/Sn/In = 0/10/90) C. Ex. 2 Mixed powder of ZnO (Al/Zn = 0/100) andITO (Al/Sn/In = 0/10/90) C. Ex. 3 50/50 ZnO 0/100 In₂O₃ 0/0/100 C. Ex. 450/50 AZO 40/60 Al-ITO 40/6/54Notes:*¹⁾ITO: Tin-containing indium oxide*²⁾AZO: Zinc oxide elementarily substituted by aluminum(Zn_(1−x)Al_(x)0)*³⁾Al-ITO: Tin-containing indium oxide elementarily substituted byaluminum<Evaluation>(Volume Resistivity of Powder)

The volume resistivities of the particles obtained in Examples andComparative Examples were evaluated. The volume resistivity was measuredby the four-probe method, using RORESTA PA system (MCP-PD41 modelmanufactured by Mitsubishi Chemical Corporation). The conditions for themeasurement were as follows:

Density of particles: 2.7 g/cm³

Inner diameter of a container: 2 cm

Distance between probes: 3 mm

(Surface Resistivity of Coating Film)

The surface resistivities of the conductive coating films of theconductive sheets obtained in Examples and Comparative Examples wereevaluated according to JIS K7194-1994. The surface resistivity wasmeasured by the four-terminal method, using RORESTA PA system (MCP-PD41model manufactured by Mitsubishi Chemical Corporation).

(Transmittance of Coating Film)

To evaluate the wavelength dependency of the transmittance, the spectrumof the transmittance of the conductive film obtained in Example 5 withina wavelength range of 200 to 2,500 nm was measured with aspectrophotometer (“Ubest V-570 model” manufactured by Jasco). Thisspectrum of transmittance is shown in FIG. 4.

Further, the spectral of the transmittance of the conductive films ofthe conductive sheets obtained in Examples and Comparative Examples weremeasured in the same manner as above within a wavelength range of 200 to2,500 nm, and the values of the transmittance of the coating films at awavelength of 350 nm were read from the spectrum of transmittance.

Table 2 shows the results of the above measurements. According to Table2, the smaller the value of the volume resistivity, the higherconductivity the particles have, which indicates the excellence of theresultant coating film as a conductive film. Table 2 further shows that,the smaller the value of the UV transmittance (indicated by“transmittance at 350 nm” in Table 2), the higher UV-shielding effectthe coating film has. Therefore, a coating film smaller both in volumeresistivity and UV transmittance has superior characteristics. TABLE 2Composite indium oxide particle[(Zn_(1−y)Al_(y)O)]_(1−x)[(ITO)_(1−z)Al_(z)]_(x) Content of ZnO Contentof Zn Content of Al pH during Volume Surface or AZO (1 − x) in particlein particle hydrothermal Particle resistivity resistivity Transmittance(mol %) (mol %) (mol %) treatment size (nm) (Ωcm) (Ω/□) at 350 nm (%)Ex. 1 50 50 0 10 20-30 9.9 × 10⁻¹ 1.3 × 10⁶ 0.05 Ex. 2 50 45 5 10 20-305.4 × 10⁻¹ 5.6 × 10⁵ 0.03 Ex. 3 20 18 2 10 20-30 1.8 × 10⁻¹ 8.1 × 10⁴1.15 Ex. 4 50 50 2 10 20-30 5.1 × 10⁻¹ 5.2 × 10⁵ 0.06 Ex. 5 50 45 7 1020-30 2.4 × 10⁻¹ 1.7 × 10⁵ 0.02 Ex. 6 20 18 5.2 10 20-30 9.8 × 10⁻² 4.1× 10⁴ 1.21 Ex. 7 50 50 0 10 50-60 1.12 2.5 × 10⁶ 0.05 Ex. 8 50 45 7 540-50 8.2 × 10⁻² 1.5 × 10⁴ 0.12 Ex. 9 20 18 5.2 5 40-60 7.3 × 10⁻³ 5.2 ×10³ 1.05 C. Ex. 1  0 0 0 10 40-70 1.5 × 10⁻¹ 4.3 × 10⁴ 86.7 C. Ex. 2 5050 0 — 20-70 1.6 × 10¹ >10⁷ 1.54 C. Ex. 3  50* 50 0 10 20-30 7.6 × 10⁵>10⁷ 0.03 C. Ex. 4 50 30 40 10 20-50 8.1 >10⁷ 0.83Note)*The composite particle of Comparative Example 3 contained no tin, sincethe composition thereof comprised (In₂O₃)_(1/2) but not ITO.

As can be seen from Table 2, the composite indium oxide particlesobtained in Examples exhibit UV-shielding effects because of the zincoxides contained therein, as compared with the tin-containing indiumoxide particles obtained in Comparative Example 1.

In addition, the composite indium oxide particles obtained in Examplesdo not show so significant difference in UV-shielding effect, but showremarkably higher values in conductivity, as compared with the simplymixed particles of the tin-containing indium oxide particles and thezinc oxide particles obtained in Comparative Example 2, the compositeindium oxide particles containing no tin obtained in Comparative Example3, and the mixture of the aluminum oxide particles and the compositeindium oxide particles obtained in Comparative Example 4. This isbecause, since the indium oxide and the zinc oxide are not compoundedbut are independently present in the particles of Comparative Example 2,the zinc oxide as an insulating substance hinders the electricalconduction among the particles, with the result that the conductivity ofthe particles is inferior to the composite indium oxide particlesobtained in Examples.

Regarding Comparative Example 3, the indium oxide is not substituted bytin, and therefore, the indium oxide as a coating substance does notexhibit conductivity, which leads to no conductivity of the resultantparticles.

Regarding Comparative Example 4, the amount of aluminum used for thesubstitution is too large, and therefore, an excess of aluminum depositsas aluminum oxide and impairs the conductivity of the particles. On theother hand, in any of the composite particles of Examples, thetin-containing indium oxide coats the zinc oxide to inhibitheterogeneous particles from hindering the electrical conduction, andthus, electrical conduction is possible among the particles by allowingan electrical current to passing along the tin-containing indium oxideon the surfaces of the particles. Further, by the substitution with anappropriate amount of aluminum, it becomes possible for the compositeparticles of Examples to maintain the conductivity to be equal to orhigher than that of the conventional simple tin-containing indium oxideparticles (Comparative Example 1).

1. A composite indium oxide particles which has an average particle sizeof 5 to 100 nm and which contains tin and zinc.
 2. A composite indiumoxide particle which has an average particle size of 5 to 100 nm andwhich comprises zinc oxide and tin-containing indium oxide.
 3. Acomposite indium oxide particle which has an average particle size of 5to 100 nm and which comprises zinc oxide and tin-containing indium oxidemaintaining crystalline structures, respectively, characterized in thatthe volume resistivity of said particles compressed under a pressure of14.7 MPa (150 kgf/cm²) is 10⁻³ to 10 Ωcm.
 4. The composite indium oxideparticle according to claim 2 or 3, wherein zinc oxide is coated withtin-containing indium oxide.
 5. The composite indium oxide particleaccording to claim 2, wherein the content of zinc oxide is 5 to 70 mol%.
 6. The composite indium oxide particle according to claim 2, whereinthe content of tin in the tin-containing indium oxide phase is 3 to 30mol %.
 7. The composite indium oxide particle according to claim 2,wherein a part of a metal element contained in at least one of zincoxide and tin-containing indium oxide is substituted by at least oneelement selected from the group consisting of aluminum and gallium. 8.The composite indium oxide particle according to claim 7, wherein thecontent of at least one element selected from the group consisting ofaluminum and gallium is 0 to 30 mol %.
 9. The composite indium oxideparticle according to claim 7, wherein a part of the zinc atom in zincoxide is substituted by at least one element selected from the groupconsisting of aluminum and gallium, and wherein the content of thesubstituting element is 0 to 30 mol % based on the mole of the zincelement in the zinc oxide phase.
 10. The composite indium oxide particleaccording to claim 7, wherein a part of the tin atom or the indium atomin tin-containing indium oxide is substituted by at least one elementselected from the group consisting of aluminum and gallium, and whereinthe content of the substituting element is 0 to 10 mol % based on thetotal mole of the tin element and the indium element in thetin-containing indium oxide phase.
 11. A process for manufacturingcomposite indium oxide particles, comprising the steps of: adding a zinccompound which comprises zinc as a main component to an aqueous alkalinesolution containing a tin salt dissolved therein, adding an aqueoussolution of an indium salt to the aqueous solution of the tin saltcontaining the zinc compound to form precipitates containing a hydroxideor hydrate of tin and indium, adjusting a pH of a suspension containingthe precipitates to 4 to 12, treating the suspension by heating at atemperature of 110 to 300° C. in the presence of water, filtering anddrying the resultant product, treating the product by heating at atemperature of 300 to 1,000° C. in an air, and treating the product byheating at a temperature of 150 to 400° C. in a reducing atmosphere. 12.The process according to claim 11, wherein at least one compoundselected from the group consisting of aluminum compounds and galliumcompounds is added to the aqueous solution of the tin salt whichcontains the zinc compound, in the step of adding the aqueous solutionof the indium salt to said aqueous solution of the tin salt whichcontains the zinc compound.
 13. A conductive coating compositioncomprising the indium oxide particles according to claim 1, a binder anda solvent.
 14. A conductive coating film formed by applying theconductive coating composition according to claim 13, and drying theresulting coating layer.
 15. A conductive sheet comprising a sheet-formsubstrate, and a transparent conductive coating film formed on thesubstrate, wherein the conductive coating film comprises the compositeindium oxide particles according to claim 1.